Such a hydraulic steering device is known from U.S. Pat. No. 5,819,532.
Such a hydraulic steering device having an amplification flow path has advantages in that not the whole flow which is supplied to the steering motor over the working port arrangement, has to pass the fluid meter means. Therefore, the fluid meter means can be kept smaller which makes it easier to use this flow meter means as an auxiliary pump in an emergency case. The flow through the main flow path which is metered by the flow meter means and the flow through the amplification flow path have a fixed relation to each other. When they are equal, i.e. the ratio is 1:1, the volume flow through the main flow path is doubled. When the volume flow through the amplification flow path is double the volume flow through the main flow path, the total volume flow is tripled.
The hydraulic steering device of the kind mentioned above has in many cases a spool and a sleeve which are rotatable relative to each other and are arranged in a bore of a housing. When an operator turns a steering wheel the sleeve is rotated relative with respect to the spool (or the other way round). This rotation opens some orifices and closes some other orifices. The open orifices allow hydraulic fluid to pass through the main flow path and through the amplification flow path. The hydraulic fluid flowing through the main flow path flows through the flow meter means. When the flow meter means is driven by the hydraulic fluid it turns back the sleeve relative to the spool into a neutral position so that the orifices in the main flow path and in the amplification flow path close again.
In some cases the flow meter means can be blocked or jammed because of dirt. The hydraulic fluid flowing through the main flow path can no longer flow freely through the main flow path. The flow of hydraulic fluid through the main flow path is stopped although the orifices in the main flow path are open. However, the amplification flow path still allows hydraulic fluid to pass from the pressure port to the working port arrangement thereby actuating the steering motor connected to the working port arrangement. Since the flow meter means is not driven by the hydraulic fluid through the main flow path there is no action to close the orifice or orifices in the amplification flow path. This leads to the risk of a dangerous situation since the steering motor is actuated in an uncontrolled way.